1. Field of the Invention
The present invention relates to a reseat system of a touch signal probe installed in a coordinates measuring machine. More specifically, it relates to a reseat system of a touch signal probe having a fixed component and a movable component, the reseat system allowing displacement of the movable component relative to the fixed component when a force is applied to the movable component from the outside and accurately returning the movable component to a rest position when the force applied to the movable component ceases to exist.
2. Description of Related Art
A touch signal probe is used in a coordinates measuring machine for detecting contact. In the coordinates measuring machine having the touch signal probe, a probe movable in three-dimensional directions touches a workpiece on a fixed table and a coordinate value of respective axes (respective axes in the three-dimensional directions) when the probe touches the workpiece is read as an electric trigger, so that dimensions and configuration of the workpiece are measured based on the coordinate values. Accordingly, a position of the probe can be detected by an electric touch signal based on contact between the probe and the workpiece.
FIG. 5 shows a conventional touch signal probe. In the figure, a stylus 1 is fixed to a movable component 2. A contact ball 4 is provided at a distal end of the stylus 1. Three cylindrical bodies 3 radially projecs with 120 degree intervals around an axis of the stylus 1 from a periphery of the movable component 2 on a plane perpendicular to the axis of the stylus 1. On the other hand, the fixed component 5 has three pairs of cylindrical bodies 6 at positions corresponding to cylindrical bodies 3 of the movable component 2. The cylindrical bodies 3 and the cylindrical body 6 constitute a reseat component for defining the relative position of the fixed component 5 and the movable component 2 at one place.
According to the above arrangement, the movable component 2 is pressed to the fixed component 5 by virtue of a biasing force F of a biasing component (not shown) and the movable component 2 is forcibly brought into contact with the fixed component 5 through the reseat component. When the pressing force from the workpiece is not applied to the distal end of the stylus 1, the movable component 2 rests on the fixed component at six contact points. In other words, respective cylindrical bodies 3 of the movable component 2 rest on the respective cylindrical bodies 6 at two points for a total of six points. Accordingly, the reseat system is called as a six-point contact reseat system.
According to the six-point contact reseat system, the reseat position of the movable component after an escape movement can be located at only one place. In other words, assuming that the stylus 1 displaces parallel to axial direction at the rest position of the stylus 1 while maintaining contact between the reseat component on the movable component side and the reseat component on the fixed component side at the respective contact points, the respective loci drawn by the distal end of the stylus crosses the axis of the stylus at the rest position. According to the arrangement, the stylus 1 returns to a unique rest position only by restoring contact with the respective contact points by the biasing force F during return movement after an escape movement of the movable component 2 by the pressing force from the workpiece, so that the rest position of the stylus 2 can be kept constant.
Since the position of the movable component relative to the fixed component can be set unique by the six-point contact reseat system, the six-point contact reseat system has high anti-vibration rigidity. Further, irrespective of the direction of the outside pressing force, the six-point contact reseat system has high reseat ability in a relatively rough unit of, for instance, 10 xcexcm.
However, the above-described six-point contact reseat system causes an error (xe2x80x9creseat shift errorxe2x80x9d) in a further fine unit of, for instance, 1 xcexcm observed in return movement after contact, the error being caused because the movable component is pushed by the workpiece during escape movement of the movable component to cause displacement relative to the fixed component.
Specifically, as shown in FIG. 6(A), when the contact ball 4 touches the workpiece W in the conventional reseat system, the stylus 1 moves in the left direction in the figure as shown in FIG. 6(B). At this time, a small reaction force is caused between the movable component 2 and the fixed component 5, so that the movable component 2 slightly slides in the left direction in the figure. When the workpiece W and the stylus 1 connected to the movable component 2 are no more in contact with each other as shown in FIG. 6(C), the movable component 2 conducts the return movement by virtue of the biasing force F, where the axial position of the movable component 2 is shifted on account of the aforesaid slide movement. The shift directly affects on measurement accuracy of the probe.
The Applicants of the present invention have proposed a reseat system capable of correcting reseat position shift after return movement shown in FIG. 7 (European Patent Publication No. 0764827 A2), where the reseat error is corrected by a piezoelectric element for administrating the direction of the friction force applied to a contact point between the movable component and the fixed component of the reseat system.
The reseat system has a fixed component 11, a movable component 21 and a biasing force generator (not shown) capable of allowing displacement of the movable component 21 relative to the fixed component 11 when a force is applied to the movable component from the outside and capable of returning the movable component 21 to a rest position when the force is not applied to the movable component 21.
The movable component 21 has a stylus 22 having a contact ball 24 to be in contact with the workpiece projecting therefrom and three cylindrical bodies 23 extending radially around the axis of the stylus 22 at 120 degree intervals to be in contact with the fixed component 11.
A central portion of the fixed component 11 is secured to a housing of the probe (not shown), the fixed component 11 having three arms 12 extending radially around the axis of the stylus 22 at 120 degree intervals. A pair of hard balls 13 is disposed on an upper surface of an end of the respective arms 12.
Further, a piezoelectric element 14 as a displacement generator is provided on an inner portion relative to the hard balls 13 of the respective arms 12, the piezoelectric elements being stretchable radially approximately along the axis of the stylus 22.
When a voltage is applied to the respective piezoelectric elements 14, the respective piezoelectric elements 14 synchronously displace, so that the respective hard balls 13 displace in approximately radial direction around the axis of the stylus 22. Incidentally, the displacement in the present arrangement is a kind of xe2x80x9cstaticxe2x80x9d displacement, which is different from vibration where the movement of the piezoelectric elements is minutely repeated.
The direction of the friction force at respective contact points between the cylindrical body 23 and the hard ball 13 aligns by the displacement, so that the reseat position can be adjusted to return the movable component by the biasing force.
However, in the above-described mechanism, though xe2x80x9caxial shiftxe2x80x9d, i.e., the reseat shift error in axial direction of the cylindrical body 23 can be effectively corrected, xe2x80x9ccircumferential errorxe2x80x9d, i.e., the reseat shift error in a circumferential direction around the axis of the stylus 22 cannot be sufficiently corrected.
Specifically, as shown in FIGS. 8(A) (seen from an upper direction in FIG. 7) and 8(B) (seen from an outside on the axis of the cylindrical body 23), when the movable component 21 conducts the return movement after the stylus 22 touches the workpiece, the cylindrical body 23 can be shifted in the circumferential direction around the axis of the stylus 22 to be supported by only one of the hard balls 13. When the cylindrical body 23 is displaced in the axial direction while being shifted in the circumferential direction, the cylindrical body 23 slides in the axial direction while keeping shifted in the circumferential direction as shown in FIGS. 8(C) to (H), so that the circumferential shift cannot be sufficiently eliminated.
The above phenomenon is thought to be caused because the effect of the biasing force for eliminating the circumferential direction is blocked by strain energy on a surface of the hard balls 13.
Specifically, minute elastic deformation is generated at the contact point P1 between the cylindrical body 23 as the reseat component on the movable component side and the hard ball 13 as the reseat component on the fixed component side on both the cylindrical body 23 side and the hard ball 13 side. Immediately after the stylus 22 is no more in contact with the workpiece and the movable component 21 conducts the return movement, there is dispersion in the direction of the elastic deformation at respective contact points P1a and P1b between the cylindrical body 23 as the reseat component on the movable component side and the hard ball 13 as the reseat component on the fixed component side and the force applied to the respective contact point. Though slightly, the elastically deformed portion receives relative slide of the movable component 21 and the fixed component 11, thus blocking return movement by the biasing force.
When displacement voltage is synchronously applied to the respective piezoelectric elements 14, the respective hard balls 13 move to project and retreat in a direction radial to the axis of the stylus 22 relative to the rest position thereof. Accordingly, the cylindrical body 23 temporarily slides in an axial direction relative to the hard ball 13. And the contact point on the cylindrical body 23 sequentially moves from P1a in FIG. 8(A) to P2 in FIG. 8(C), P3 in FIG. 8(E) and P4 in FIG. 8(G). Therefore, the direction of the elastic deformation at the contact points and the force applied to the contact point can be leveled on the cylindrical body 23 side, thus eliminating the axial shift.
However, as shown in FIGS. 8(B), 8(D), 8(F) and 8(H), the contact point P1b on the hard ball 23 stays at one point irrespective of the slide of the cylindrical body 23, and the elastic deformation cannot be leveled. Accordingly, the strain energy for preventing the cylindrical body 23 from returning in the circumferential direction still works on the contact point P1b on the hard ball 23 side, which is thought to be a reason for the circumferential shift not to be eliminated. Since the circumferential shift still remains, the reseat shift cannot be corrected with extremely high accuracy.
An object of the present invention is to provide a reseat system of a touch signal probe capable of highly accurately correcting reseat shift error after the return movement of the movable component.
A reseat system of a touch signal probe according to the present invention includes: a fixed component; a movable component having a stylus; a first reseat component provided on the fixed component; a second reseat component provided on the movable component, the second reseat component touching the first reseat component at a pair of contact points on three locations mutually spaced apart, the reseat system of a touch signal probe allowing displacement of the movable component relative to the fixed component when outside force is applied to the stylus and returning the movable component to a rest position when the outside force is not applied to the stylus by virtue of a biasing force; and a contact point displacer for changing both of the contact point on the fixed component and the contact point on the movable component for at least a predetermined distance.
According to the present invention, during return movement of the movable component, both of the contact point on the fixed component and the contact point on the movable component change position thereof. Accordingly, neither one of the contact point on the fixed component or the contact point on the movable component keeps elastic deformation during displacement as contrary to the conventional arrangement, so that the strain energy by the elastic deformation is eliminated by the displacement. Therefore, a force for preventing return to the rest position does not work on any one of the contact points on the fixed component and the movable component, so that the reseat shift error can be effectively corrected.
In the above-described reseat system of a touch signal probe, the predetermined distance may preferably be larger than the Hertzian elastic deformation caused on the contact points on the fixed component and the movable component.
Accordingly, since the amount of movement of both of the contact points on the fixed component and the movable component exceeds the Hertzian elastic deformation, the strain energy can be sufficiently eliminated by the elastic deformation, so that shift correction function can be effectively performed.
In the above arrangement, the contact point displacer may preferably include: a curved surface formed on one of the first reseat component and the second reseat component; a slant surface formed on the other of the first reseat component and the second reseat component, the slant surface slanting relative to a radial direction of an axis of the stylus; and a drive source for relatively displacing the first reseat component and the second reseat component.
According to the present arrangement, both positions of both of the contact points on the movable component and the fixed component can be changed by a single drive source. Therefore, the construction of the reseat system can be simplified.
Specifically, the first reseat component or the second reseat component may preferably be a pair of hard balls and the other may preferably be a cylindrical body having conic outer circumference.
Alternatively, the first reseat component or the second reseat component may preferably be a pair of cylindrical bodies arranged in a V-shape and the other may preferably be a cylindrical body having a conic outer circumference.
Further alternatively, the first reseat component or the second reseat component may preferably be one hard ball and the other may preferably be a V-shape groove having a cut surface slanting relative to the radial direction of the axis of the stylus.
According to the above configuration, the present invention can be achieved by only changing a part of a component of the conventional reseat system. Therefore, the conventional production facility, product parts and production process can be applied to the production of the present invention, thus avoiding significant increase in the production costs. Further, a component having high accuracy can be easily manufactured, thereby easily implementing the effect of the present invention.
In the above, the contact point displacer may preferably include a displacement generator for relatively displacing the first reseat component and the second reseat component on respective contact points between the first reseat component and the second reseat component while keeping contact between the movable component and the fixed component after the outside force is ceased to be applied to the movable component to finally return the movable component to the rest position, the displacement generator also serving as the drive source.
In the present invention, the circumferential shift can be corrected during the return movement even without the displacement generator by changing positions of both of the contact points on the fixed component and the movable component during, for instance, the minute slide movement in returning the movable component to the rest position. In this case, the drive source is a mechanism for returning the movable component to an initial posture.
Further, at least the first reseat component or the second reseat component may be rotated to change the positions of the contact points on both of the fixed component and the movable component without relatively displacing the fixed component and the movable component. In this case, the rotary mechanism serves as the drive source.
However, by providing the displacement generator for artificially causing relative displacement of the fixed component and the movable component after the return movement thereof, the circumferential shift can be corrected with high accuracy by a simple structure.
In the above, it is preferable that the displacement generator relatively displaces the respective fixed components and the respective movable components along only a single direction.
Though the above-described present invention can be implemented by providing piezoelectric elements for displacing in a plurality of different directions, the structure can be complicated. According to the above arrangement, the effect of the present invention can be achieved by providing only one displacement generator for each combination of the fixed component and movable component. Accordingly, the structure of the reseat system can be simplified without largely increasing production costs thereof.